At least 20,000 children are trapped in the Islamic State stronghold of Fallujah which is under siege by Iraqi forces and faces severe shortages, the UN Children's Fund said on Wednesday.

"UNICEF estimates that at least 20,000 children remain trapped in the city," the agency's Iraq representative Peter Hawkins said, adding that they face forced recruitment as militants.

The few residents who have managed to flee the Fallujah area since Iraqi forces launched an offensive against the Islamic State (IS) group on 22-23 May speak of severe shortages of food and drinking water.

The few hundred families who have escaped IS territory were camped out on the outskirts and residents reached by phone inside the city spoke of even worse conditions.

"Children face the risk of forced recruitment into the fighting, strict procedures for security screening and separation from their families," Hawkins said.

"Children who are recruited see their lives and futures jeopardised as they are forced to carry and use arms, fighting in an adult war."

UNICEF renewed calls for safe corridors to be opened to allow Fallujah's civilian population to escape.

The United Nations has accused IS of using the civilian population as human shields as it prepares to mount a desperate defence of one of its most emblematic bastions.